I've used these outlets for several years now. If I recall correctly, I got them from Autozone. They have a spring loaded lid on them (they're made to be installed on the exterior of a vehicle, like the bed of a truck). When you plug a device into it, the spring loaded lid pushes against the side of the device's plug, which puts pressure on it and helps keep it in place. For added security, you can buy some velcro tape (the kind you find at a hardware store), and wrap it around the lid and the device's plug to keep everything from coming loose.
It's not as good as a lock in powerlet, but it's a heck of a lot cheaper. I've ridden all over the country with them, with no issues. For a USB device, I just plug a USB adapter into the outlet.
The brackets are homemade.
"No matter where you go, there you are."